Short answer: work in this order. Set the lighting class (for example EN 13201 P2 = 10 lux average), convert it to lumens and LED watts, apply a dimming profile, then size for the worst month’s sun. Panel watts = daily Wh ÷ (peak sun hours × 0.75). Battery = daily Wh × (1 + backup nights) ÷ 0.9 for LiFePO4. Finally, check the panel can refill the battery after a cloudy spell.
Most solar street light failures are sizing failures, not product failures: the LED is oversized for the road, the panel is too small for the rainy season, or the battery is empty by the second cloudy night. This method avoids all three.
Key numbers
- EN 13201-2 average illuminance for P classes (streets, footpaths, parking): P1 15 lux, P2 10, P3 7.5, P4 5, P5 3, P6 2.
- Typical design autonomy in practice: 3–5 backup nights, more for critical roads and cloudy climates. IEC 62124 tests autonomy and recovery, but does not set a minimum.
- Kenyan county tenders we reviewed asked for LiFePO4 batteries with ≥ 4,000 cycles and warranties of up to 5 years.
- In our worked example, an 80 W panel would need about 16 sunny days to refill the battery after three cloudy nights; a 120 W panel needs about 4.
1. Start with the light the road needs
The lighting class comes from the road type and the local standard. EN 13201-2 and CIE 115 use the same P-class values for residential streets and pedestrian areas:
| Class | P1 | P2 | P3 | P4 | P5 | P6 |
|---|---|---|---|---|---|---|
| Average illuminance (lux) | 15 | 10 | 7.5 | 5 | 3 | 2 |
| Minimum illuminance (lux) | 3 | 2 | 1.5 | 1 | 0.6 | 0.4 |
A first estimate of lumens uses the lumen method: lumens = target lux × pole spacing × road width ÷ (utilisation factor × maintenance factor). For a 7 m road, 25 m spacing and P2 (10 lux), with a utilisation factor of 0.5 and maintenance factor of 0.8: 10 × 25 × 7 ÷ 0.4 ≈ 4,400 lm. At an assumed 150 lm/W system efficacy, that is a 30 W LED, not the 60–100 W often written into specifications. Confirm with a DIALux layout using the actual photometric (IES) file.
2. Daily energy, with a dimming profile
Lights rarely need full output all night. A common profile for 12 hours of darkness:
| Period | Hours | Output | Energy for a 30 W LED |
|---|---|---|---|
| Dusk to 22:00 | 4 | 100 % | 120 Wh |
| 22:00 to 02:00 | 4 | 60 % | 72 Wh |
| 02:00 to dawn | 4 | 30 % | 36 Wh |
| Total per night | 228 Wh | ||
Motion sensors can raise output when someone passes. Count that as extra energy, not as a saving.
3. Peak sun hours for the worst month
Peak sun hours (PSH) are the daily solar energy on the panel expressed as hours of full 1,000 W/m² sun. Size for the worst month of the year at the panel’s tilt, not the annual average. The World Bank’s Global Solar Atlas and NASA POWER give monthly values for any location. Many equatorial and Sahel sites design at 4.5–6 PSH; humid coastal and monsoon regions can fall much lower in the rainy season.
4. Panel size, then check recovery
Panel watts = daily energy ÷ (PSH × 0.75). The 0.75 allows about 25 % for temperature, dust, wiring and controller losses. For 228 Wh at 4.5 PSH: 228 ÷ (4.5 × 0.75) ≈ 68 W, so the next standard size is 80 W.
That panel balances an average day, but it cannot recover after bad weather. The surplus on a sunny day is what refills the battery:
| Panel | Harvest per sunny day (× 4.5 PSH × 0.75) | Surplus after 228 Wh load | Days to refill 3 cloudy nights (684 Wh) |
|---|---|---|---|
| 80 W | 270 Wh | 42 Wh | about 16 |
| 100 W | 338 Wh | 110 Wh | about 6 |
| 120 W | 405 Wh | 177 Wh | about 4 |
This is our own calculation and assumes no harvest on cloudy days, so it is conservative. It explains why lights that pass a sunny-week test go dark in the rainy season: the battery never gets back to full. For this example we would specify 120 W.
5. Battery and backup nights
The battery must cover tonight plus the number of cloudy nights you want to ride through. With LiFePO4, plan on using about 90 % of nominal capacity: battery Wh = 228 × (1 + 3) ÷ 0.9 ≈ 1,010 Wh, which is a 12.8 V, 80 Ah pack. Use three backup nights as a minimum for LiFePO4 on ordinary streets, and four to five for main roads, critical routes or cloudy climates.
6. Check tender specifications the same way
Tender line items are often written separately and do not balance. Two examples from Kenyan tenders we reviewed:
- A split system with a 150 W LED and a 1,240 Wh battery. With the dimming profile above, the light uses about 1,140 Wh a night, so the battery covers one night and almost no cloudy weather.
- An integrated light with an LED over 100 W, a 120 W panel and a 700 Wh battery. It only works with much deeper dimming than our profile.
Neither is wrong as such, but the bidder needs to state the dimming profile and the resulting autonomy. Otherwise the client compares lights that will behave very differently in the first rainy season. See our Kenya tender checklist.
Summary of the worked example
| Step | Calculation | Result |
|---|---|---|
| Lumens for P2, 7 m road, 25 m spacing | 10 × 25 × 7 ÷ (0.5 × 0.8) | about 4,400 lm → 30 W LED |
| Energy per night | Dimming profile | 228 Wh |
| Panel, balance | 228 ÷ (4.5 × 0.75) | 68 W |
| Panel, with recovery | Refill 3 nights in about 4 days | 120 W |
| Battery | 228 × 4 ÷ 0.9 ÷ 12.8 V | 80 Ah LiFePO4 |
What specifications often miss
- Panel tilt and orientation. Face the equator at roughly the site latitude, plus about 10–15° for better worst-month yield and self-cleaning in the rain.
- Shading from trees and buildings, which can cut the energy the panel collects sharply.
- Battery temperature. Hot battery boxes shorten life; most LiFePO4 cells must not be charged below 0 °C. Ventilation, shading and a controller with temperature protection matter.
- Theft and vandalism, which decide where the battery goes. See our anti-theft design guide.
FAQ
How many days of autonomy should a solar street light have?
Three backup nights is a common minimum for LiFePO4 on ordinary streets; four to five for main roads, critical routes or cloudy climates. Ask for verification by an IEC 62124-type test of autonomy and recovery.
What peak sun hours should I use?
The worst month at the panel’s tilt, from the Global Solar Atlas or NASA POWER, not the annual average.
Why do solar street lights dim after midnight?
Dimming when traffic is low can cut energy use by a third or more, which lets a smaller panel and battery give the same reliability. The profile should be stated in the bid.
Integrated or split-type for higher wattages?
Split systems allow a larger panel at the right tilt and a bigger battery, so they suit main roads and higher outputs. Integrated lights are simpler to install for lower outputs.
How we help
Try the numbers for your site with our solar lighting calculator, or send us the road width, pole spacing, lighting class and location. We will size the LED, panel and LiFePO4 battery, state the dimming profile and autonomy, and propose matching poles.
Sources: EN 13201-2:2015 Table 3 and CIE 115:2010 P-class values, as reviewed by Fotios (University of Sheffield, 2019); IEC 62124:2004, Photovoltaic stand-alone systems, design verification; World Bank Global Solar Atlas and NASA POWER for solar data; Kenyan county tender documents (2025–2026) for battery and warranty requirements. Recovery and sizing figures are our own worked example. Related: our solar street lights, LED street lights.